As technology development and demand for mobile devices are increasing and the spread of electric vehicles is expanding, demand for secondary batteries as energy sources is rapidly increasing. Among them, demand for lithium secondary batteries having high capacity and energy density is especially high.
Generally, a lithium secondary battery is fabricated by manufacturing an electrode assembly composed of a negative electrode, a positive electrode, and a separator, inserting the electrode assembly into a battery case, and injecting an electrolyte into the electrode assembly. The lithium secondary battery thus produced is required to be activated by a predetermined charge and discharge to function as a battery. Such a process is referred to as a formation process or an activation process. The secondary battery is also shipped after an aging process and a defective product sorting process. The aging process is an aging process in which the electrolyte is allowed to enter the empty space of the electrode to allow time for forming a stable electrolyte channel.
The secondary battery is manufactured so that a positive electrode and a negative electrode are prevented from being contacted by a porous insulating film (separator) to prevent a short circuit. However, insulation may not be properly maintained due to various reasons during the manufacturing process of the battery. As a result, an internal short circuit of the battery can occur. Lithium-ion batteries can be ignited or exploded when a positive electrode and a negative electrode are short-circuited. Even when they are slightly short-circuited, ions move and current flows. This condition is often referred to as a soft short or a micro short.
Soft-shorts cause low-voltage defects. Soft-short cells tend to take a relatively long time to be expressed as compared to hard-short cells, and their expression time vary considerably depending on the short-circuit state or degree.
Recently, a method of selecting a low voltage failure of a secondary battery in an activation process has been used to select a failure of the secondary battery. However, during the activation process, it is determined that the secondary battery is normal. However, the secondary battery may become further defective during the storage period until the secondary battery is inserted into the battery pack after the activation process is completed. In addition, a product to be judged as a low-voltage defect may be erroneously determined as a normal product and may be put into a battery pack together. Particularly, although the low voltage defect due to the soft short is measured by measuring the voltage fluctuation width through the aging process after the manufacture of the battery, it is difficult to select the low voltage defect within a short period of time. If the low voltage defect does not occur within a predetermined period, the defective products will be supplied.
In the conventional defective sorting process, although a difference in voltage drop between good and defective products is used, there is a problem that failure cannot be detected properly when the time for low voltage expression takes a long time. However, it is undesirable to perform the aging process for a long time until a low voltage failure is manifested because it decreases the productivity.
Therefore, if the time required for low-voltage expression can be predicted and utilized in the aging step, particularly in the shipping aging step, the unnecessary aging time can be shortened to thereby improve the productivity, and the defect detection rate can be increased to prevent the supply of defective products.